An API is an Application Programming Interface and services often have APIs available for developers to write software which accesses information offered by the service.
An API is an Application Programming Interface offered by a service. They allow developers to write software which interacts with the service in some way. APIs are often used for developers to retrieve information quickly and easily for use in their own software. APIs can also be used to control accounts on a service. In Bitcoin, many APIs are used to retrieve blockchain data, price data, or control web wallets.